tyke ā Leapfrogging back a little, āSā is known as an alternative version of the =S= mark of John Stonier & Co., etched on to the glass at the Stuart factory. It's not common; I've only seen two or three examples.
Stonier's certainly supplied glass to the two original Cunard Queens, Elizabeth and Mary, and boxed sets were used as competition prizes, see here
. BTW that wavy line decoration was a Cunard exclusive, not specifically for RMS Queen Mary.